1. Field of The Invention
The invention relates to electromagnetic clutches and, more particularly, to an improved magnetic housing for an electromagnetic clutch.
2. Description of the Related Art
A conventional electromagnetic clutch is described, for example, in U.S. Pat. No. 4,694,945. Such a conventional electromagnetic clutch is shown in FIG. 1 and includes magnetic pulley 30 which is rotatably mounted on tubular extension 1a of compressor housing 1 through bearing 2. Magnetic pulley 30 is rotated by an external power source through a belt (not shown). Electromagnetic device 400 is disposed in a stationary position in a hollow portion defined by pulley 30 and is secured on compressor housing 1 through support plate 10. Hub 6 is secured on the outer terminal end of drive shaft 5. Armature plate 8 is supported on hub 6 by a plurality of leaf springs 7. Armature plate 8 faces the axial end surface of pulley 30 with a predetermined axial gap therebetween and, through the resiliency of leaf springs 7, is capable of limited axial movement.
In the above described electromagnetic clutch, pulley 30 includes outer cylindrical portion 31 on which belt receiving grooves 31a are formed, inner cylindrical portion 32, and axial end plate portion 33 connecting the outer and inner cylindrical portions 31 and 32. Annular hollow portion 34 is defined by cylindrical portions 31 and 32 and axial end plate portion 33. Outer and inner cylindrical portions 31 and 32 are connected to axial end plate portion 33 through step portion 35 and 36, respectively. Stationary electromagnetic device 400 is placed within hollow portion 34.
Electromagnetic coil 21 is fixedly disposed within an annular hollow space 23 of annular electromagnetic housing 401 by insulating resin 22. Annular electromagnetic housing 401 has a U-shaped cross section and is fixed to supporting plate 10 by spot welding. Housing 401 comprises outer and inner cylindrical portions 401a and 401b connected by axial annular end plate portion 401c. Annular electromagnetic housing 401 is secured to the axial end surface of housing 1 by a snap ring 9. A small air gap is maintained between annular electromagnetic housing 401 and pulley 30.
Axial end plate portion 33 is provided with concentric slits 33a and 33b to form a plurality of concentric annular magnetic pole faces. A plurality of annular shaped grooves 8a and 8b are formed on the axial end surface of armature plate 8 opposite axial end plate portion 33 of pulley 30. Annular grooves 8a and 8b face concentric slits 33a and 33b. Armature plate 8 also includes slit 81 placed midway between annular shaped grooves 8a and 8b. As a result, armature plate 8 is divided into four pole portions by slits 33a and 33b and grooves 8a and 8b, so that magnetic flux zigzags between axial end plate portion 33 and armature plate 8.
In the above arrangement, when electromagnetic coil 21 is energized, magnetic flux, as indicated by arrows, is produced and flows through a closed loop including axial annular end plate portion 401c of electromagnetic housing 401, inner cylindrical portion 401b, inner cylindrical portion 32, outer bearing race 2, step portion 36, armature plate 8, axial end plate 33, step portion 35 and outer cylindrical portion 31. Armature plate 8 is attracted to axial end plate portion 33, so that drive shaft 5 is rotated together with pulley 30 in a manner similar to conventional electric magnetic clutches. When electromagnetic coil 21 is de-energized, armature plate 8 separates from pulley 30 and returns to its original position due to the recoil strength of leaf springs 7. Therefore, pulley 30 continues to rotate although the electromagnetic clutch is disengaged.
The magnetic attraction between pulley 30 and armature plate 8 is related to the magnetic flux flowing through the closed loop. It is advantageous for the magnetic reluctance of the closed loop to be relatively low. The magnetic reluctance of the loop is directly proportional to the sectional area of the magnetic flux flowing through the closed loop. Therefore, the sectional area of axial annular end plate portion 401c of electromagnetic housing 401 should be larger than that of outer and inner cylindrical portion 401a and 401b of electromagnetic housing 401 because most of the magnetic flux flows through axial annular end plate portion 401c in comparison with outer and inner cylindrical portions 401a and 401b.
In today's smaller cars, the space in which the engine and accessories, such as automotive compressors, are mounted has become smaller. Nevertheless, while the size of the compressor is reduced, its capacity is still required to be sufficient enough to cool the passenger compartment. To achieve this objective, the diameter of pulley 30 could be reduced. This may increase the rotation speed of the pulley 30. In this reduced diameter pulley, the distance between inner diameter D1 of outer cylindrical portion 31 and outer diameter D2 of inner cylindrical portion 32 decreases. Moreover, as a result of such a reduction, the distance between concentric slits 33a and 33b also decreases. This in turn reduces the frictional contact surface between armature plate 8 and pulley 30. This reduction, however, has the disadvantage of reducing the transmitted torque. To overcome this reduction in torque transmission, the axial end surface of the axial end plate portion 33 has step portions 35 and 36 formed on the outer peripheral portion of axial end plate portion 33 for increasing the contact surface area between pulley 30 and armature plate 8. Therefore, the outer diameter of outer cylindrical portion 31, on which belt-receiving grooves 31a are formed, can be made smaller than the outer diameter of axial end plate portion 33, so that high rotating speeds can be accomplished without too much reduction of torque transmission.
Referring to FIGS. 2(a)-2(b), a method of producing electromagnetic housing 401 is disclosed in accordance with the above described electromagnetic clutch. Electromagnetic housing 401 according to the prior art is formed as a single, unitary body of magnetic material. First, a magnetic circular plate A of a predetermined dimension with a central opening is prepared. Magnetic circular plate A is then press-drawn to form annular blank B (FIG. 2(b)) having outer annular cylindrical portion 401a, inner annular cylindrical portion 401b, and axial end annular plate portion 401c. Blank B then is subjected to a reverse drawing or upset working to press outer and inner cylindrical portions 401a and 401b. Thus, electromagnetic housing 401 forms a hollow portion in which electromagnetic coil 21 is disposed. Cover plate 10 is fixed to the end surface of axial end plate portion 401 c by brazing. Further, the inner end of cover plate 10 is secured to an axial end surface of housing 1 by snap ring 9.
If, during the press-drawing step, the relative dimensions between D3 and D4 and H (FIG. 2(b)) are not adequately established, outer annular cylindrical portion 401a, inner annular cylindrical portion 401b, and axial end annular plate 401c may be formed with differing thicknesses. These variations may become especially apparent in axial annular end plate 401c. Consequently, during the press-drawing step, the thinner portions of blank B may be partly crushed. Preferably, the relative sizes of D1, D2 and H may be approximated from the following equation: ##EQU1##
Magnetic circular plate A should be sufficiently thick, e.g., about 5 mm, so that axial end plate 401c has a small magnetic reluctance therein. Due to this increased thickness of magnetic circular end plate A, however, more heat is generated between the press-drawing die and the work piece during the press drawing step. If enough heat is generated, the work piece bonds to the press-drawing die. To prevent this, the work piece may be bathed with a metallic soap treatment before the press-drawing step. Further, the material cost of a magnetic circular plate A is relatively high because a relatively thick but malleable material must be employed. Consequently, the prior art electromagnetic housing 401 requires sophisticated manufacturing process techniques and astute material selection.